Monosize poly(methyl methacrylate-co-butyl acrylate) rubbery nanoparticles, approximately 95 nm in diameter, containing iodine and triiodide with a molar ratio of 2.78 of the former to the latter, were synthesized by emulsion polymerization. Iodine and the two-monomer mixture were polymerized by sodium metabisulfite and potassium persulfate as a redox initiation system. Iodine evolution to triiodide during the copolymerization was traced by ultraviolet-visible spectroscopy. Iodine addition to the monomer mixture and its copolymerization not only reduced the copolymer average molecular weight by 30%, but also led to the formation of more MMA-enriched chains. The redox initiation system also changed 26% of initial iodine molecules into triiodide, while the rest were encapsulated as molecular iodine. During thermal annealing, the lower rate of triiodide decomposition into iodine and iodide in comparison with the iodine evaporation rate broadened both thermogravimetric and heat flow curves.